1. Field of the Invention
The present invention relates generally to an improved catalyst for use in Fischer-Tropsch processes. More particularly, the present invention relates to a method of improving the structural integrity of a Fischer-Tropsch catalyst without losing substantial catalytic activity and selectivity toward heavy hydrocarbons. Still more specifically, the present invention relates to a method of producing a Fischer-Tropsch catalyst containing a structural support such as a binder incorporated after precipitation of the catalyst precursor or a support material coprecipitated with iron. The support material increases the structural integrity of the catalyst. The catalyst of the present disclosure may comprise coprecipitated material selected from iron, silica, magnesium, copper, aluminum, and combinations thereof. Alternatively, or additionally, potassium silicate binder, colloidal silica, and/or tetraethyl ortho silicate (TEOS) may be added to a precipitated catalyst to increase the strength thereof.
2. Background of the Invention
The Fischer-Tropsch (FT) technology is used to convert a mixture of hydrogen and carbon monoxide (synthesis gas or syngas) to valuable hydrocarbon products. Often, the process utilizes a slurry bubble column reactor (SBCR). The technology of converting synthesis gas originating from natural gas into valuable primarily liquid hydrocarbon products is referred to as Gas To Liquids (GTL) technology. When coal is the raw material for the syngas, the technology is commonly referred to as Coal-To-Liquids (CTL). The FT technology is one of several conversion techniques included in the broader GTL/CTL technology.
One of the primary difficulties encountered in using iron-based catalysts for carrying out the FT reaction in a slurry bubble column reactor (SBCR) is the breakdown of the initial catalyst particles into very small particles, i.e. less than 5 microns in size. Although the small particle size is advantageous for increasing surface area and reaction rate of the catalyst, the problem lies in separating the small catalyst particles from the wax slurry medium. Separating the catalyst particles from the wax is necessary since the iron catalyst when operated under the most profitable conditions wherein wax is produced requires removal of the wax from the reactor to maintain a constant height of slurry in the reactor.
There are at least three modes of iron catalyst breakdown. First, when the catalyst undergoes activation, the starting material, hematite, is converted to iron carbides which have different structures and density. The induced stresses from the transformation lead to particle breakage. Second, if the reactor is operated at high temperature, e.g. greater than about 280° C., or at low H2:CO ratio, e.g. less than about 0.7, carbon formation via the Boudouard reaction can pry the particles apart. Third, mechanical action can cause breakup of the particles due to catalyst particles impinging each other or the reactor walls.
It is impossible to determine the actual attrition resistance required without knowing the type of reactor system, the type of wax/catalyst separation system and the system operating conditions.
Heretofore, attempts at developing strengthened iron-based catalysts have focused on producing the strongest possible catalysts, regardless of the actual strength required for a particular system. Such approaches sacrifice activity and selectivity for catalyst strength which may exceed that which is required. Most of this work has focused on attempting to maximize strength of the catalyst without due regard for the negative impact of high levels of strengthener, e.g. silica, on activity and selectivity. Further, tests for catalyst strength have been carried out ex-situ, i.e. outside the SBCRs. Many of the tests have been conducted in a stirred tank reactor (autoclave) which subjects the catalyst to severe shearing forces not typically encountered in slurry bubble column reactors.
Improved catalyst strength can be achieved by depositing the iron on a refractory support such as silica, alumina or magnesia or by adding a structural promoter to the baseline catalyst. The challenge is to strengthen the catalyst without appreciably compromising the activity and selectivity of the catalyst. Use of binders, for example, SiO2 binder, has been performed at high levels, e.g. 10%-15%. These catalysts seem to yield very light products. Silica (SiO2) and alumina (Al2O3) as supports at high levels (˜10%) are known8,9.
In a paper entitled “Attrition of precipitated iron Fischer-Tropsch catalysts” by Dinesh S. Kalakkad, Mehul D. Shroff, Steve Kohler, Nancy Jackson, and A. K. Datye (Applied Catalysis A: General 133 (1995) 335-350), attrition of a precipitated iron catalyst promoted with copper and potassium was studied. The catalyst was prepared by United Catalyst (now Sud-Chemie). It was reported that the low agglomerate strength of this catalyst led to attrition on the micron scale caused by physical action on the catalyst. Phase transformations and carbon deposition that accompanied exposure of the catalyst to carbon monoxide at elevated temperatures were found to cause break-up of the catalyst particles into nano-scale carbide particles.
Hien N. Pham, Alexander Vierguyz, Robert J. Gormley, and Abhaya K. Datye published a paper entitled “Improving the attrition resistance of slurry phase heterogeneous catalysts” (Powder Technology 110 (2000) 196-203) wherein ultrasound was used to test the attrition resistance of precipitated iron catalyst precursors with and without the addition of silica as a binder. Attrition resistance was determined by measuring the particle size distributions before and after ultrasonic treatment. By adding potassium silicate to the precipitated Fe/Cu precursor to achieve a 25 weight % silica loading, a significant improvement in attrition resistance was reported compared to the spray dried Fe/Cu precursor.
In a paper entitled “Preparation of Attrition Resistant Spray-Dried Fe Fischer-Tropsch Catalysts Using Precipitated SiO2” (Ind. Eng. Chem. Res. 40 (22), (2001) 4778-4784) by Kandis Sudsakom, James G. Goodwin, and K. Jothimurugesan, it was reported that the addition of more than 10-11% silica to an Fe/Cu/K catalyst lowered the attrition resistance. A correlation was reported between attrition resistance and catalyst particle density. As silica is added to the iron catalyst, the particle density decreases. It was reported that this decrease in particle density correlated with an increase in catalyst attrition resistance.
Accordingly, there is a need for a catalyst and a method of making same which has improved resistance against breakdown and also maintains the salient features of an unsupported iron catalyst, viz. high activity and selectivity toward high molecular weight hydrocarbons. Such a catalyst should preferably also improve separation of the catalyst from the reaction mixture.